The present invention relates to improvements in electrochemical cells, particularly cells having negative electrodes comprising zinc (Zn) particles, such as in alkaline batteries.
An electrochemical cell (i.e., a galvanic cell or battery) has the following basic components: a negative electrode (sometimes called an anode), a positive electrode (sometimes called a cathode), and an ion-conductive solution (sometimes called an electrolyte) providing a path for the transfer of charged ions between the two electrodes when they are connected through an external load.
Some alkaline cells have anodes with zinc as an active element, and cathodes with manganese dioxide (MnO.sub.2) as an active element. Anodes do not have to be solid; in fact, conventional alkaline cells have a gelled zinc anode mixture. The mixture contains individual zinc metal particles suspended in a thickened liquid or gel containing a gelling agent, an alkaline electrolyte such as potassium hydroxide (KOH), and minor amounts of other additives, such as indium or bismuth (gassing inhibitors for reducing the undesirable tendency for hydrogen gas to build up inside the cell). The zinc particles are characterized by a specific size range, commonly indicated by the standard mesh size through which the particles pass. Typically, average anode particle sizes fall in the range of about -50/+200 mesh, indicating particles that pass through a 50 mesh screen and do not pass through a 200 mesh screen (the larger the screen number, the smaller the aperture size of the screen).
Common gelling agents used in anodes include carboxymethycellulose, polyacrylic acid (e.g., Carbopol 940.TM. from B.F. Goodrich in Brecksville, Ohio, or POLYGEL-4P.TM. from 3V in Bergamo, Italy), sodium polyacrylate (e.g., CL-15.TM. from Allied Colloids in Yorkshire, England), and salts. Non-limiting examples of gassing inhibitors include inorganic additives such as indium, bismuth, tin and lead and organic inhibitors such as phosphate esters and anionic and non-ionic surfactants. See U.S. Pat. Nos. 5,283,139, 5,168,018, 4,939,048, 4,500,614, 3,963,520, 4,963,447, 4,455,358, and 4,195,120 for examples of various anode mixtures.
The gel anode is typically separated from the cathode by a separator, such as a thin layer of non-woven material or paper, that prohibits electronic conduction between the anode and the cathode but allows ions to pass between them.
Alkaline Zn/MnO.sub.2 cells have been commercially available for over 30 years, during which time their performance characteristics have been incrementally optimized by the industry in an attempt to provide the "longest lasting" battery (i.e., one with the greatest overall capacity, measured in ampere-hours) within the volume constraints imposed by the international size standards (e.g., AAA, AA, C, D cylindrical and 9 volt prismatic sizes). The volume within such standard cells, into which the active materials are packed, is more or less fixed. The amount of energy available from any given cell size (which is a function of the total amount of the active elements in the cell) has a theoretical upper limit which is defined by the internal cell volume and the practical densities of the active components that are employed.
In addition to trying to produce the "longest-lasting" battery, battery manufacturers are also trying to increase the maximum instantaneous rate of electrical current that can be generated from a battery under a given load without the battery voltage dropping below a minimum value. The motivation for increasing this "maximum discharge rate" capability includes the ongoing development of electronic products, such as cellular phones, which require high currents from small packages. Some of these new devices automatically test the voltage levels of their batteries, and therefore may cause the premature disposal of batteries which have remaining overall capacity, if the sensed voltage dips excessively during a period of high current draw.
When a high current is being drawn from a battery, the voltage of the battery may drop due to zinc particle surface "passivation" or anode polarization which can indicate a localized lack of sufficient hydroxide ions to sustain the chemical reaction of the cell. It is believed that a certain amount of porosity is necessary for the free supply of OH.sup.- ions coming from the electrolyte and the free disposal of Zn(OH).sub.4.sup.-, Zn(OH).sub.2 or ZnO reaction products back into the electrolyte. If the zinc particles are too densely crowded, or if their surfaces are inaccessible due to accumulation of reaction products, the reaction cannot keep up with the rate of current draw. Batteries with densely packed zinc in their anodes may perform acceptably with very stable voltage levels while supplying low continuous currents, but drop to very low, unacceptable voltages when a high current is drawn due to zinc crowding (sometimes referred to as "choking" or being "electrolyte starved").
In addition, too little electrolyte can starve the overall chemical reaction of the cell or cause the battery to "dry out", as water from the electrolyte is continuously consumed during discharge. The net reaction inside the cell is:
Zn+2MnO.sub.2 +H.sub.2 O.fwdarw.ZnO+2MnOOH.
Thus, competing with the desire to pack as much zinc as possible into the available anode volume to increase overall capacity for "long life" is the need to provide a sufficient amount of electrolyte to avoid "choking" during periods of high discharge rate.